Hydraulic control device



July 29, 1941.

H. W. LANQENBERGER HYDRAULC CONTROL DEVICE 2 sheets-sheet 1 Filed March l5, 1958 July 29, 1941.

H. W. LANDENBERGER HYDRAULIC CONTROL DEVICE 2 Sheets-Sheet 2 Filed' March 15, 1958 T \n. Q Nm O i ,mn

WW .k w w o n M u w 00W --"7 o o o Hm v NN n rl www ' tion of the pump at a wide latehted July 29, 1941 signor to American Engineering pany, l Pahliadelphia, Pa., a corporation o! Pennsylv a application March 15, 193s, sensi No. 195,957 (ci. s-.ssi

10 Claims.

This invention relates to hydraulic control devices, and more particularly to devices for controlling the pressures developed by pumps.

One object of the present invention is to provide in a fluid system including a variable stroke pump, adjustable control means adapted to condition the pump for operation at any one of a wide range of initial predetermined pressures, to

then conditionA the pump for operation at any one of a wide range of diiferent predetermined pressures, and thereafter to cause a gradual change in pressuresdeveloped by said pump until the initial predetermined pressure has been attained.

' Another object is to provide control means for pumps of the stated character, including adjustable resilient means adapted to condition the pump for operation at any one of avwiderange of predetermined pressures, and adjustable iiuid pressure means operable upon said resilient means for conditioning the pump for operation at any one of a wide range of diierent predetermined pressures...

A further object is to provide a control mechanism for pumps of the stated character including a novel arrangement of valves, one of which permits free flow of fluid in one direction, but restricted ow in the reverse direction.

A further object is to provide in a fluid system including a pump and a motor, control means adapted to condition the pump for operation at any one of a wide range of initial pressures during a portion of the travel of said motor'in one direction, to then condition said pump for operation at any one of a wide range of difterent predetermined pressures, and when said motor reaches its limit of travel to gradually redu'ce the pressures developed by said pump until` the initial predetermined pressure has been attained. .L c A further object is to provide in -a iluid system including a pump and a motor, a control device including a novel arrangement of the automatically operable valves which enables operarangepi predetermined pressures. l

Other and further objects will become apparent as the description of the invention progresses.

0f the drawings:

. Fig. 1 is ageneral assembly view, diagrammatic in part, of the fluid control device comprising the' present invention, several of the elements thereof being shown in section. j

Fig. 2 is a side elevational view partly in sec- `tion of the pump and the controly means therefor,

certain of the parts being broken away to show certain of the elements thereof, and

Fig. 3 is a fragmentary view of one end of the pump and certain of the control elements associated therewith.

Referring to the drawings, the numeraLI designates generally the uid system which in the present instance comprises a pump 2, a

hydraulic press 2, press control valve 4, pump\ control valves 5 and 6, and iluid motors 1 and 8.

Pump 2 may be of the radial piston type shown and described in `Patent No. 1,077,979, and'may beA operated by an electric motor 9 or by any other suitable power means. Pump 2 and motor 9 are suitably mounted upon a supply tank I0 from which iluid is drawn into the pump 2 through a pipe II. The iluld so drawn into pump 2 is discharged 'therefrom through a pipe I2. Pipe I2 is connected at one end thereof m the inlet port I3` of press control valve 4. Valve 4 one end of a pairl of pipes I9 and I9. 'I'he other ends of pipes I8 and I9 are connected to the lower and upper ends respectively, of the cylinder 20 of a fluid motor 2l secured in anysuitable manner to the frame 22 of hydraulic press I. The opposite ends of chamber I5 are connected by a passage 23 provided in casing I4 which in turn communicates with a discharge port 24. Discharge port 24 is connected to supply tank I0 by means of a pipe 25. Operating in chamber I5 of valve 4 is a valve element 2B comprising a pair of spaced interconnected valve heads 27 and 28. A valve stem 29. is connected to valve head 2l and extends through the lower end of casing I4.` The free end of valve stem 29 is pivotally connected to a lever 3 9 intermediate the ends of the latter. Lever. 30 is pivoted at one end thereof to the lower end of a link. 3|, the other end of said link being pivotally mounted on an ear I2 secured to and extending laterally from the lower end of casing I4. When lever I0 is in the full'line position shown in Fig. 1, the fluid from pump 2 flows to the lower end of cylinder 24 through pipe I2, port I3 of valve 4, chamber IB. port Il and pipe I8. The piston 33 operating in cylinderA 29 is therefore moved upwardly. The iluld at the upper end of cylinder 20 now discharges to exhaust through pipe I9,- port I1, the lower end of chamber I5, passage 23, and pipe 25. When lever 30 is moved to the dotted line position shown in Fig. 1, the iluid entering chamber I5 of valve 4 flows through port I1 and is conducted to the upper end of cylinder 20 by pipe |9, thereby causing the piston 33 to descend. The iluid at the lower end of cylinder 20 now flows to supply tank I0 through pipe I8, port I6', the upper end of chamber I5, passage 23 and pipe 25. Piston 33 has secured thereto a piston rod 34, to the upper end of which is se- 'cured a platen 35. Consequently when piston 33 is moved upwardly platen 35 is actuated toward the work which may be attached to the head 36 of press 3. 'Ihe head 36, as shown in Fig. 1, is i secured to the upper end of the frame 22 and extends laterally therefrom.

The present invention contemplates a control device which enables operation of the pump at a relatively low predetermined pressure at which pressure the press platen begins its initial movement toward the work. When the platen engages the work the pressure in the system builds up to a higher value. The present invention further contemplates means for maintaining a predetermined higher pressure in the system and to thereafter gradually reduce the pressure until the initial, relatively low, predetermined pressure has been reached. The control mechanisms for accomplishing these results will now be described in detail.

Referring more particularly to Fig. 2, it will -be observed that a bracket 40 is secured to the left hand side of pump 2 by any suitable means, such as by bolts 4|. The iupper endl of bracket 48 y has provided therein the cylinder 42 ofv fluid motor 8. Secured to the opposite side of pump 2 i by bolts' 43 is a second bracket 44. The lower end of this bracket has formed therein the cylinder\45 of the fluid motor 1. The vertical extension 46 ofA bracket 44 has provided therein a horizontally disposed aperture 41 through which extends a rod 48. The 'left hand end of -rod 48 extends through the central bore 49 of a nut 58 which makes threaded engagement with the enlarged portion 5| of cylinder 42. Rod 48 has slidably mounted thereon a crosshead 52 against which one end of a coil spring 53 abuts. Spaced from crosshead 52 and secured to rod 48 by any suitable means, such as a screw 54', is an adjustable spring seat 54 against which the other end of spring 53 abuts. Crosshead 52 has provided at the opposite ends thereof a pair of rectangular guide blocks 55 and 56, which yrespectively .receive the upper bifurcated ends 51 and 58 of a pair of levers 59 and 68. Levers 59 and 68 are pivoted intermediate their ends on a rod 6|, mounted in any suitablemanner in `bracket 44. The lower bifurcated ends 62 and 63 of levers 59 and 68 engage a pair of substantially rectangular bearing blocks 64 and 65 pivotally mounted on guide rods 66 and 61, respectively. The left hand ends of guide rods 66 land 61 are secured to the sliding blocks 68 of pump 2, while the right hand ends thereof are secured in any suitable manner to a crosshead 69, such as by nuts 18, engaging the outer threaded ends of said guide rods. Cylinder 45 of fluid motor 1 has operating therein a piston 1|, the right hand end (Fig. 2) of which engages the inner side of crosshead 69 at a point substantially midway between guide rods 68 and 61. 'I'he left hand end of cylinder 45 communicates with a substantially vertical port 12 provided in the lower end of bracket 44, and to which one end of a pipe 13 is connected. The other end of pipe 13 is connected to pipe |2 by means of a T coupling 14, and a short pipe 15.

causes an axial movement of rod 48. After proper adjustment of spring -53 lthe nuts 11 may be held against accidental movement by a set screw 11', or by any other suitable means. If the nuts 11 are so adjusted as to cause rd 48 to move to the right, spring seat 54 will tend to compress spring 53. The pressure thus created `in spring 53 tends to slide crosshead 52 to the right which in turn swings levers '59 and 68 in a clockwise direction about their pivot 6|. This action of levers 59 and 60 causes the sliding Iblocks 68 of pump 2 to move to the left by virtue of the connection thereof with guide rods 66 and 61. The sliding blocks 68 of pump 2 will remain in adjusted position until the pressure imposed upon the left hand end of piston 1| exceeds the pressure of spring 53. When this occurs piston 1| will move to the right, thereby actuating crosshead 69, guide rods 66 and 61, and consequently the sliding blocks 68 to leakage stroke position.

The cylinder 42 of fluid motor 8 also has operating therein a piston 88. The right hand end of piston extends into the enlarged bore 8| at vthe left hand end (Fig. 2) of nut 50, and is adapted to engage the left hand end of rod 48. It will be noted that the diameter of piston 80 is greater than that of rod 48, and that the movement of piston 88 to the right is limited by the annular shoulder 82 at the right hand end of bore 8|. It therefore is seen that when piston 88 is subjected to pressure it tends to move to the rightand thereby compresses spring 53. By adjusting nut 58 the extent of movement 'of piston 80, and consequently the degree of compression of spring 53, may be varied. Motor 8 is also connected to the pressure side of pump 2 by means of a series of connections which will now be described in detail. A,

Cylinder 42 of motor 8 has secured to the left hand end thereof a plug 83 having a central port 84 provided therein. The outer end of port 84 is connected to the port 85 of valve 6 -by means of a pipe 86. The port 81, at the opposite side of valve 6, is connected to the intermediate port 88 of valve 5 by means of a pipe 89. The inlet port 90 of valve 5 is connected to the T coupling 14 by means of a pipe 9|. Thus when the valves 5 and 6 are in the proper position of adjustment, fluid under pressure from pump 2 is conducted to the end of motor 8, through pipes I2, 9|, valve 5, pipe 89, valve 6, pipe 86 and port 64.

Referring more particularly to Fig. 1, it will be observed that valve 5 comprises a casing 92 having a substantially cylindrical chamber 93 provided centrally thereof. Mounted in chamber 93 is a valve element 94 comprising a'pair of spaced interconnected valve heads 95 and- 96. Valve head 95 has secured thereto a reduced extension 91 to the outer end of which is secured a further reduced valve stem 98. Valve stem 98 extends through and is guided in an aperture left hand end |09. l t Y Y 'I'he valve B is of the type which permits sub' vided in the right hand end wall |04 of casing 92 and the right hand end thereof is secured to' thereto an arm |09 constructed of any suitable insulating material. 'I'he left hand end of arm |09 has provided at the lower side thereof a contact plate ||0, and at its upper side a, second contact plate Lower contact plate ||3 is adapted to engage a pair of contact elements ||2 and |I3 secured to a bracket ||'4 mounted on a vertically disposed rod I I5 secured in any suitable manner to the press frame 22. Bracket ||4 may be adjusted vertically and held inany position of adjustment by a set screw H3. 'Ihe upper contact plate III is adapted to engage a second pair of contact elements ||1 and |I3 secured .to a second bracket ||9 also mounted on rod II5. Bracket ||9 may also be adjusted vertically and held in any position of adjustment by means of a set screw |20. Contact ||2 is connected to one end of solenoid |09 by a conductor Contact I I3 on the other hand is connectedy vm and m 'provided therein. casing no has to a bus bar |22, leading from any suitable source g nected to bus bar |22 by a second conductor |21.

The other end of solenoid |0| is connected to bus bar |24 by a third conductor |28. The circuit just described may be closed or interrupted by means of a switch |29. It therefore is seen that Y l when switch |29 is closed and the parts are in the position shown in Fig. 1, the circuit to solenoid |0|i is closed, thereby causing actuation of the valve 94 to the right. The movement of the valve 94 tothe right is limited by the' end of the extension |02 abutting the end Wall |04 of I the valve casing. When the parts are v`in this position fluid from pipe |2 entersl the central chamber 93 of valve 5 between valve heads 95 and 96 through pipes 9| and port. 90. From chamber 93 the iiuid then flows through port 33 and into pipe 99, which conducts it'to the port 91 of valve 9.

If valve 4 is adjusted to the position shown.l in Fig. 1,*uid alsoeenters the lower end of cylinder 20 causing the piston 33 to move upwardly. ,AsA

'soonaspiston 33 beginsits' upward movement contact plate ||0 moves out of engagement with '.-contactelements ||2 and ||3,.thereby interrupting'the circuit to solenoid |06 of valve 5'. Valve element 94, however, being of the balanced type, remains in adjusted position. When the platen 35 reaches its upper limit of travel contact plate lllengages contacts I1 and ||9, thereby closing the-circuitto'solenoid |0I. When this occurs' valve element 94 is -actuated to its left hand position of adjustment whereby the iiuid entering chamber 93-is trapped between valve heads 95 and-99. Fluid from motor 9, as will more fully appear` hereinafter, may then flow to exhaust through pipe 99, valve 9, pipe 99, port 33, the ofvchamber 93, port- |01-and pipe stantiallyfree flow of uid in the direction of reverse direction and comprises a casing having .aA pairof relatively movable-valve elements |33. Valve element |3| is provided with an annular flange |33 at the upper end thereof, which, when inits lower position, rests upon a valve seat |31. Valve .|3I is normally held upon its seat |31 by a coil spring |39, which extends between the upper surface of said valve arid an adjustable plug |39, making threaded engagement with an aperture provided in the upper end of casing '|30. Valve |3| is also provided with a`central bore |40 through which the lower end of valve element |32 extends. The-portion' of valve |3| which extends through passage |35, it will be noted,` is tapered, thus when said valve is moved upwardly fluid from chamber |34 may pass around the outer surface thereof and into chamber |33.

The upper end of valve element |32 has secured thereto a threaded shank |4| which engages the correspondingly threaded central bore of plug |33. Thus, valve element |32 may be raised or lowered by simply turning the shank |4| thereof. To prevent accidental movement of valve element |32, a protective cap |42 is provided over the end thereof andsecured to the shank |4|-by making threaded contact therewith. Valve element |32has provided therein a wedge-shaped slot |43 through which iiuid'rom chamber |33 may pass to chamber |34., The rate at which iiuid may pass through yslot |43 depends upon the position of valve |32. When the valve is in its lowermost position substantially no fluid can pass through the valve. However, as the valve element |32 is elevated, progressively greater quantities of uid may pass therethrough.v The lower chamber |34 of valve19 isconnected to port 31 by .a passage |44. Consequently the iluid entering valve 9 throughport 91 will now ow to chamber |34 through passage |44. The iiuid pressure so entering valve 9 now operates against` the lower endof valve element |3I, thereby lifting the same from .its seat and,- permitting a substantially freeilow of iiuid through passage |35 to chamber |33. From chamber |33 the fluid` flows-,through port 95 to pipe 99, which conducts it to cylinder 42.01' motor 9.' I f the uid pressure so directed against the end of piston is suffi-` cient to overcome the pressure 'exerted by spring 1 53' the `said piston moves :to the right (Fig. 2), thereby further compressing spring 53.4 Piston 93'moves to the right until. the end thereof engages the annular shoulder 92 provided in, the

adjustablenut 90.v When valve element 94 is |33 from cylinder 42 now leaks into chamber |34 through'the slot |43'provided in valve element |32, and then iiowsto exhaust through passage |44, port 31, pipe 99, port 83, the right hand end of chamber 93, port |01 and pipe |09.

When the pressure operating on piston 90 of l fluid motor 3 ,is relieved, spring 53moves rod 43, and consequentlypiston 90 to thev left until the parts assume their initial position of adjustment. This action of the spring 53 upon the parts forces the fluid through valve 9 in the manner just described. Due to the restricted flow of fluid through valve element' |32, a retarded action results, and'consequently the pressure developed by pump 2 gradually decreases.

secured thereto by nuts |46. as shown in Figs.

1 and 2.4 'As shown more particularly in Fig. 2, any leakage of fluid past piston 38 drains to supply tank i3 through a pipe I4l.

From the foregoing description it is thought that the operation of my improved hydraulic control device is apparent. However, in order to correlate the various elements thereof a brief rsum of the operation of the apparatus as a l whole will now be given. A

Fig. 1 shows the parts in normal position wherein the piston 33 and platen 35 are in their lower position of adjustment. Brackets ||4 and ||9 are now adjusted vertically to suit the size of the materials to be placed in the press 3. When piston 33 is in its lower position of adjustment,

the contact plate |I8 is in engagement with con-l tact elements ||2 and ||3, thereby completing the circuit to solenoid |33. Nuts. I'l are now adjusted to compress spring 53 to condition the pump for operation at the desired predetermined initial pressure. This causes actuation of levers 53 in a clockwise direction,` which in tum through their connection with rods 66 and 31 actuate sliding blocks 58 of the pump to on-stroke position. Nut 53 is then adjusted the' proper amount to enable further compression of spring 53 by piston 38. Valve element |32 of valve 5 is also adjusted to regulate the rate of discharge therethrough. Lev'er '33 is then moved to the lull -lin position shown in Fig. 1.

The preliminary adjustments to the apparatus having been completed, switch |29 is now closed, thereby completing the circuit to solenoid |33 and causing operation of valve element 94 to its right hand position of adjustment, as shown in Fig. 1. Fluid now is drawn into pump 2 upon operation thereof by motor 9 from supply tank I3 through pipe II, and is discharged therefrom through pipe |2. Fluid then enters the central chamber I of valve 4 through port I3 and then flows through port i3, and pipe I3, to the lower end of cylinder 23, causing piston 33, and consequently platen 35 to move upwardly toward the work. The fluid in-cylinder 23, above piston 33,`now passes to supply tank I8 through pipe I9, port I1, of valve 4, the lower end ofI chamber I5, passage 23,

4port 24, and pipe 25. Upon movement of piston 33 upwardly contact plate ||0 moves out of engagement with contact elements ||2 and ||3 thereby interrupting the circuit to solenoid |33. Valve 5 being of the balanced type, however,

-remains in adjusted position. Fluid from pipe I2 also passes through pipe 9|. port 93 of valve 5, to the central chamber 93 thereof, and then discharges from chamber93 to'port 33, and enters port 81 of valve 5 through pipe 89." The fluid entering port 31 of valve 8 passes through passage |44 to chamber |34 and operates against the lower surface of valve element |3|, causing the latter to move upwardly away from its seat against the tension of spring |38. The fluid now passes to the central chamber |33 through the communicating passage |35 and then ows through port 35 and into pipe 35 which conducts it to the end of cylinder 42 of iiuid motor 3.

As the platen 35 engages the work, thek pressure in the system increases, and when this pressure operating on the'end of piston 30 exceeds the compression of spring 53 piston 83 moves to the right, causing further compression of spring 53. Piston 3|| continues its movement to the right until the right hand end thereof (Fig. 2) engages the annular shoulder 32 of nut 50. Pump 2 is now conditioned to operate at a higher predetermined pressure. The pressure in the system is also delivered to the left hand end (Fig. 2).'of fluid motor I through pipe 13, and when this pressure exceeds the pressure of spring 53 piston II actuates crosshead 39, .and consequently the sliding blocks 53 of pump 2 to leakage stroke' position. The relatively high predetermined pressure is thus maintained at a constant value.

As platen 35 reaches its upper limit of travel, contact plate engages contact elements ||1 and |I3, thereby completing the circuit to solenoid IUI, which actuates valve element 94 to its left hand position of adjustment.

Fluid-from pump 2 entering the central chamber 93 of valve 5 through pipes I2 and 9|' is now trapped between valve heads 95 and 96.

When valve element 94 is adjusted to this position. valve head 95 uncovers port 38 of valve 5 whereby the said port now communicates with the right hand end of chamber 93, which .in turn is connected to supply tank ||I by pipe |08. Cylinder 42 of motor 3 is now connected to exhaust, the fluid therein discharging through port 34, pipe 33, port 35, to the central chamber |33 of valve 5.. The uid then flows through the slot |43 of valve element |32 and into lower chamber |34. A

The rate at which fiuid discharges through slot |43 depends upon the initial setting of valve element |32. The fluid entering chamber |34 then iiows through passage |44, port 31, pipe 39, port 88, to the right hand end of 'chamber 93 of valve 5 from which it discharges to supply tank I3 through port |81 and pipe |03. Valve element |32 acts as a check valve to prevent a rapid discharge of the fluid from motor 8, and consedelivers fluid pressure in accordance with the initialsetting of spring 53 by the adjustment of nut V1'I. This initial pressure is maintained by motor 1 which, it will be remembered, is con nected to the pressure side of pump 2 by pipe 13. Should the pressure tend to exceed this pre -1 determined value piston II moves to the right toad-just the element 63 toward leakage stroke position, as hereinbefore described. When platen 35 has completed its work upon the materials, the operator actuates lever 3|I of valve 4 to the dotted line position shown in Fig. 1, whereupon fluid discharging from pump! through pipe i2 now flows from central chamber I5 of valve 4 through port |'I, and is conducted to the upper end of piston 33 by pipe I9. whereupon the said piston 33, as well as platen 35, descend. The

materials may now be removed from the press.

aasdsso cylinder 20 now passes to supply tank Il through.

pipe I8, port I6, the upper end of chamber I of valve 4, passage 2,3, Dort!! and pipe 2l. When the piston 33 reaches its lowermost position, con. tact plate lili again engages contact elements H2 and H3, whereupon valve element 94 ot valve 5 is urged to the position shown in Fig. 1 by the action of solenoid Hi8. The operator then swings lever 30 of valve l to the full line pomf tion shown in Fig. l, whereupon iluid from pump 2 is now delivered to the lower end oi' piston 33, and the cycle oi' operation just described is repeated. i

From the foregoing description it is apparent that a, simplied, as well 'as highly emcientvcontrol device has been provided. The various ada justable features associated with the device enable operation thereof through awide range of pressures. The device therefore may be em- `ployed for various uses wherein such variable ,pressures are desired.

While the embodiment herein shown and described is admirably adapted to fulfill the objects primarily stated, it is to be understood that it' is not intended to limit the invention thereto,

. since it may be embodied in other formaall coming within the scope of the claims which follow.

What is claimed is:

1. In a device of the class described, the combination of a variable stroke pump, control means including manually adjustable pressure means for placing said pump on stroke, means connected to the .pressure side of said pump and operating in opposition to said pressure means for returning said pump to leakage stroke position when'the pressure acting thereon exceeds that of -4 said pressure means, and power means operable upon said pressure means for changing the initial Vadjustment of the latter whereby said pump is maintained on stroke at a diierent predetermined pressure.

2. In a device of the class described, the combination of a variable stroke pump, control'means including manually adjustable pressure means for `placing said pump'on stroke, means connected to the pressure side of said pump and operating in opposition to said 'pressure means for returning said pump to leakage stroke position when the pressure acting thereon exceeds that of said pressure means, power means operable upon` said pressure means for changing the initial adjust ment of the latter whereby said pump is maintained on stroke at adifferent predetermined pressure, and adjustable means for limiting the movement of said power means.

3. In a device of the class described, the combination of a variable stroke pump, control means including an adjustable spring for placing said pump on stroke, a fluid motor connected to the 4 pressure side of said pump and operating,in

opposition to said spring for returning said 'pump to leakage stroke positiomwhen the pressure act.- ing on said motor exceeds the pressure 0f said spring, manual means for adjusting said Spring whereby said pump may be maintained on stroke at dierent predetermined pressures, and a second uid motor connected to the pressure side oi said pump and operable upon said spring to change the pressure toV which it was originally adjustedfby said manual means, whereby 'said pump is maintainedon stroke at a different predetermined pressure. f

Y motor connected to the pressure side of said 4.` In a device of the class described, the com Vbination of a variable stroke pump, control means including an adjustable spring for placing said pump on stroke, a fluid motor connected to the 5' pressure side -of said pump and operating in opposition tosaid spring vfor returning said pump to leakage stroke position when the pressure act'- ing on said motor exceeds the pressure of said spring, manual means for adjustingsaid spring l0 whereby said pump may be maintained on stroke at different vpredetermined pressures, a second iluid motor connected to the pressure side of said l, Apump and operable upon said spring to change the pressure tn which it was originallyv adjusted 15 motor means.

5. In a device of the class described, the combination of a. variable stroke pump, control means including an adjustable spring for placing said pump on stroke, a fluid motor -connected to the pressure side -of sail pump and operating in opposition to saidsp'ing for returning saidpump to leakage stroke position when the pressure acting thereon exceeds the pressure of said spring, manual means for adjusting said spring whereby said pump may be maintained on stroke at dihferent predetermined pressures, a second iiuid pump, said` second uid motor being operable upon said spring to change the pressure to which it was originallyadjusted by said manual means, whereby said pump is maintained on stroke at a different predetermined pressure, said second motor comprising a cylinder having apiston operable therein, and means for predetermining the extent of movement of said piston, said last mentioned means comprising an adjustable nut secured to one end of said cylinder and having an i opening provided therein for receiving said piston, and stop means provided in said opening for limiting the movement of said piston. 6. In a device of the class described, the combination of a pump, control means for said pump including a spring for placing said pump' on stroke, means operating in opposition .to said spring for returning said pump to leakage stroke position .when the`pressure developed by said pump exceeds the pressure of said spring, manual means for adjusting said spring to predeter.

` mine the pressures to be developed by said pump,

- fluid pressure means including a plunger operable upon said spring for changing the initial adjusts ment thereof whereby operation of said pump liseifected at a higher pressure, valve means for controlling the operation of said plunger, said valve means comprising a three-way valve and a. x

flow control valve, said owcontrol valve comprising elements which permit free ow of fluidto said uid pressure means when said three-WayI valve is actuated to one position of adjustment,

and restricted ilow from said iiuid pressure means when said three-way valve is actuated to a sec-` ond positionL of adjustment.

7. In a device of the class described. the combinationof a pump, control means for said pump'y including a spring for placing said lpump on stroke, means operating in opposition to said spring for returning Vsaid pumpto leakage stroke position when the pressure developed by said l pump exceeds the pressure of said spring, manual means for adjusting said spring to predetermine the pressures to be developed by said pump, fluid pressure means including a plunger operable upon said spring for changing the initial adjustment thereof whereby operation of said pump is effected at a higher pressure, valve means for controlling the operation of said plunger, said valve means comprising a three-way valve and a ow control valve, said flow control valve comprising elements which permit free ilow of uid to said uid pressure means when said three-way valve is actuated to one position of adjustment, restrictedrflow from said iiuidl pressure means when said three-way valve is actuated to a second position of adjustment, and automatically operable means for actuating said three-Way valve to different positions of adjustment.

8. In a device of the class described, the combination of a pump, control means for said pump including a spring for placing said pump on stroke, means operating in opposition to said spring for returning said pump to leakage strokeposition when the pressure developed by said pump exceeds the pressure of said spring, manual means for adjusting said spring to predetermine the pressures to be developed by said pump, uid pressure means including a plunger operable upon said spring for changing the initial adjustment thereof whereby operation of said pump is effected at a higher pressure, valve means for controlling the operation of said plunger, said valve means comprising a three-Way valve and a iiow control valve, said flow control valve comprising elements which permit free iiow of fluid to. said fluid pressure means when said three-way age stroke position, means for adjusting said resilient means to predetermine the pressures to be developed by said pump, fluid pressure means connected to the pressure side of said pump and operable upon said resilient means for changing the initial adjustment thereof, means for interrupting the circ-uit between said pump and fluid pressure means, and a. valve for restricting the flow of iluid from said uid pressure means to exhaust when the circuit is so interrupted whereby said resilient means gradually returns to its initial position of adjustment.

10. In a device of the class described, the combination of a pump, control means for said pump including a spring for placing said pump on stroke, means operating in opposition to said spring for returning said pump to leakage stroke position when the pressure developed by said.

pump exceeds the pressure of said spring, means for adjusting said spring to predetermine the pressures to be developed by said pump, fluid pressure means operable upon said spring to change the initial adjustment thereof whereby valve is actuated to one position of adjustment, I

'restricted'.flow from said uid pressure means when said three-Way valve is actuated to a second position of adjustmentA and electrically operable means for actuating said three-way valve to different positions of adjustment.l

9. In a device of the class described, the combination of a pump, control means for said pump including resilient means for placing said pump on stroke, means connected to the pressure side of said pump and operating in opposition to said resilient means forreturning said pump to leakoperation of said pump is elected at higher pressure, a uid circuit connecting said uid pressure means to the pressure side of said pump,

valve means provided in said circuit including a three-Way valve and a iiow control valve, said three-way valve when adjusted to one position permitting fluid from said pump to pass to'said control valve, a pressure actuated valve element provided in said control valve which permits substantially free ilow of fluid to said uid pressure means, means for actuating said three-way valve to disconnect the flow of fluid from said pump to said fluid pressure means, while connecting the latter to exhaust, an adjustable check valve element provided in said flow control valve to re-v strict the ow of iluid from said iiuid pressure means to exhaust, whereby said spring gradually moves to its first position of adjustment causing the pressures developed by said pump to gradually decrease until the first mentioned pressure 

